Three light sources A , B , and C emit an identical amount of...

The human eye is most sensitive to yellow-green light, which has a wavelength of $555 \text{ nm}$.

The relative luminosity curve of the human eye peaks at $555 \text{ nm}$ and decreases for both shorter and longer wavelengths.

Since sources $A$, $B$, and $C$ emit the same amount of radiant energy per unit time, the brightness sensed by the eye is directly proportional to the relative luminosity at their respective wavelengths.

Because the sensitivity is maximum at $555 \text{ nm}$ (source $B$), the brightness sensed for source $B$ will be greater than that for source $A$ ($450 \text{ nm}$) and source $C$ ($700 \text{ nm}$).

Therefore, $X_B > X_A$ and $X_B > X_C$.